In the field of seismic exploration, the earth interior is explored by emitting low-frequency, generally from 0 Hz to 200 Hz, acoustic waves generated by seismic sources. Refractions or reflections of the emitted waves by features in subsurface are recorded by seismic receivers. The receiver recordings are digitized for processing. The processing of the digitized seismic data is an evolved technology including-various sub-processes such as noise removal and corrections to determine the location and geometry of the features which perturbed the emitted wave to cause reflection or refraction. The result of the processing is an acoustic map of the earth interior, which in turn can be exploited to identify for example hydrocarbon reservoirs or monitor changes in such reservoirs.
Seismic surveys are performed on land, in transition zones and in a marine environment. In the marine environment, surveys include sources and receiver cables (streamers) towed in the body of water and ocean bottom surveys in which at least one of sources or receivers are located at the seafloor. Seismic sources and/or receivers can also be placed into boreholes.
The known seismic sources include impulse sources, such as explosives and airguns, and vibratory sources which emit waves with a more controllable amplitude and frequency spectrum. The existing receivers fall broadly speaking into two categories termed “geophones” and “hydrophones”, respectively. Hydrophones record pressure changes, whereas geophones are responsive to particle velocity or acceleration. Geophones can recorded waves in up to three spatial directions and are accordingly referred to as 1C, 2C or 3C sensors. A 4C seismic sensor would be a combination of a 3C geophone with a hydrophone. Both types of receivers can be deployed as cables with the cable providing a structure for mounting receivers and signal transmission to a base station. Such cables fall into two distinct categories: one being so-called ocean-bottom cables which maintain contact with the sea-floor, while the second category is known as streamers which are towed through the water without touching the sea-floor.
Presently, the seismic industry is in the process of developing multi-component cables or streamers. Multi-component streamers include a plurality of receivers that enable the detection of pressure and particle velocity or time derivatives thereof. In so-called dual sensor towed streamers, the streamer carries a combination of pressure sensors and velocity sensors. The pressure sensor is typically a hydrophone, and the motion or velocity sensors are geophones or accelerometers. In the U.S. Pat. No. 6,512,980 a streamer is described carrying pairs of pressure sensors and motion sensors combined with a third sensor, a noise reference sensor. The noise reference sensor is described as a variant of the prior art pressure sensor.
In the United Kingdom patent application GB 0402012.9, there is proposed a streamer having a plurality of compact clusters of hydrophones. The streamer is adapted to provide gradient measurements of pressure, which in turn can be readily transformed into particle velocity data.
The main motivation for developing multi-component streamers has been to decompose the recorded data into its up- and down-going components, i.e., to free the data of “ghosts” caused by reflection at the sea surface. In this memo we introduce a new application area for multi-component streamers.
On the other hand, the seismic industry has since long experienced the need to interpolate or extrapolate trace recordings into areas void of receivers. Normally the wavefield and/or its derivatives are only known at a number of discrete locations. However, in practice it is often desirable to extend the knowledge of the wavefield to other points using interpolation, extrapolation or a combination of extrapolation and interpolation, sometimes known as intrapolation. Such techniques are applied, for example, to determine pressure data along the streamer, away from a streamer, at near-source offsets, or between two adjacent streamers.
In the light of the above prior art, it is seen as an object of the present invention to provide improved methods of interpolating and extrapolating seismic recordings.